The present invention relates to an electronic surveillance system which includes a plurality of audio and video sensors that are selectively activated from a remote command and control station.
Electronic surveillance systems have long been employed in a wide variety of security applications such as monitoring banks and other industrial and military restricted-access areas. In many of these systems, one or more television cameras and microphones are positioned to observe the area to be monitored and the sensors are connected by electric cables to a remotely located command post such as a guard or police station. The cameras often have several controllable functions such as variable lens aperture, focus and zoom, and they may be mounted on motorized pan and tilt translators. Other surveillance systems have avoided the limitations of a cable connection between the sensors and the command station by using a radio-frequency communication link of some kind.
A surveillance system in which the sensors are linked by coaxial cable to the central station is disclosed in U.S. Pat. No. 4,511,886. A plurality of video cameras and microphones are provided for monitoring a number of locations such as cash register positions. The outputs of the sensors are sampled sequentially by a video switcher and converted to compressed, single sideband signals to conserve the bandwidth required of the coaxial cable. Coding information is included in the transmitted signals for identifying which of the plural sensors is activated and unauthorized interception of the signals is prevented by randomly varying the frequency of the signal carrier.
Another surveillance system is disclosed in U.S. Pat. No. 4,326,221 in which a plurality of audio-video sensors are linked to a central station through a radio frequency transceiver. The sensors are individually addressed and controlled in response to a series of tones by which the lens aperture, focus and zoom settings of the cameras are operated. Signals received at the central station are recorded and displayed as desired and the central station may be activated by an intrusion sensor at the remote location. Since the sensors and central station are not linked by a cable, either or both may be mobile.
The surveillance systems described above are intended for permanent installations in which considerations of physical size, power consumption and reliability are of limited importance. In many security applications these become critical. The monitoring and protection of commercial aircraft is one such application where the size, weight, power and reliability of the airborne components of the surveillance system is of special concern. Also, neither of the systems already described is directed to providing security for the surveillance system itself or for the command signals transmitted to the sensors by the monitoring station. Further, it is often important that the surveillance system should not be visible to personnel in or around the area being monitored and that the system should be easily installed in an existing facility with a minimal amount of disruption. To prevent unauthorized activation of the system or interception of the surveillance information, it is also desirable that the command signals and sensor signals be encrypted